Spectroscopic and electrochemical properties of di-2-pyridyl ketone thiosemicarbazone [dpktsc] in non-aqueous media

Abstract

► Deprotonation of [dpktsc] is not favorable in the presence and absence of NaBH 4 . ► Facile deprotonation of [dpktsc] occurs in the presence of trace amounts of [MCl 2 ] (M = Zn, Cd or Hg). ► Reductive decomposition of [dpktsc] occurs in dmf. ► [MCl 2 (κ 3 -N,N,S-dpktsc-H] − is formed when [dpktsc] was allowed to interact with [MCl 2 ]. Spectroscopic measurements on non-aqueous solutions of [dpktsc] divulged high sensitivity of [dpktsc] to its surroundings. 1 H NMR studies performed on d 6 -dmso and d 6 -acetone solutions of [dpktsc] disclosed inter-molecular hydrogen bond between the amine proton and the solvent oxygen atom. In non-protophilic solvent (CDCl 3 ), intra-molecular hydrogen bond between the amide proton and a nitrogen atom of a pyridine ring was elucidated. Electronic absorption measurements done on non-aqueous solutions of [dpktsc] revealed two intra-ligand charge transfer transitions (ILCT) due to n → π * and π → π * of the thione followed by thione to pyridine charge transfer. In protophilic solvents, a shoulder appeared at ∼400 nm along with the ILCT transitions. Spectrophotometric titrations of [dpktsc] with NaBX 4 (X = H or F) and thermo-optical measurements in dmf disclosed that deprotonation of [dpktsc] is not favorable. When stoichiometric amounts of [MCl 2 ] (M = Zn, Cd or Hg) were added to dmf solutions of [dpktsc], facile conversion of [dpktsc] to its conjugate base [dpktsc-H] − and in situ formation of [MCl 2 (κ 3 -N,N,S-dpktsc-H] − was observed. Dmf solutions of [dpktsc] are able to detect and determine [MCl 2 ] in concentrations as low as 1.00 × 10 −12 M. Protophilic solutions (dmf) of [dpktsc] show high affinity to [ZnCl 2 ], compared to [MCl 2 ] (M = Cd or Hg). Electrochemical measurements done on dmf solutions of [dpktsc] in the presence and absence of [MCl 2 ] show reductive decomposition of [dpktsc] in the absence of [MCl 2 ] and in the presence of [MCl 2 ] electrochemical signatures consistent with the in situ formation of [MCl 2 (κ 3 -N,N,S-dpktsc-H)] − were observed.